Pulse signals emitted by conventional rotary dialing switches are often distorted as a result of long transmission distances, and the signals are consequently occasionally incorrectly identified. This erroneous signal phenomenon is known as Wrong-Number Interference. In 1968, CCITT recommended a set of Dual-Tone Multiple-Frequency (DTMF) signals to represent the communication signal between the subscriber and the center station. DTMF systems employ two groups of frequencies each having four distinct frequencies. Using the two groups of frequencies, there are sixteen different combinations of dual tone multiple frequency. Electronic Research and Service Organization (ERSO), a branch of Industrial Technology Research Institute (ITRI), has developed a series of DTMF generators since 1983.
As a result of subscribers' demands, attempts have been made to reduce the working voltage range of DTMF Generators downwardly from 5 volts, e.g. to 3 volts, 2.5 volts and even to 2 volts. It has been found, however, that conventional DTMF generator architecture is not capable of operating properly at such a low voltage.